With the spread of an information terminal, there has been a growing demand for a flat panel display as a computer display. Further, the information having been provided in the form of paper is now given in the form of electronic paper in ever-increasing occasions, as a result of progress in the information technology. There has been an increasing need for electronic paper or digital paper as a mobile display medium characterized by lightweight and excellent portability. Against this backdrop, development of a flat panel display apparatus has been carried out.
Generally in the flat panel display apparatus, a display medium is produced by using a device based on the technology of liquid crystal, organic electroluminescence and electrophoresis. To ensure uniformity of the screen brightness and screen rewriting speed, such a display medium is manufactured according to the technology based on an active drive element made of an organic thin film transistor (hereinafter referred to as “organic TFT”) as an image drive element.
In this case, in the production of the organic TFT element, a semiconductor thin film such as a-Si (amorphous silicon) and p-Si (polysilicon) and a thin metallic film such as a source, drain and gate electrode are sequentially formed on the glass substrate.
In the conventional practice, to form a bus line, electrode pattern and semiconductor layer on a substrate, patterning is carried out to form a circuit by the photolithographic technology (hereinafter referred to as “photolithography”). Photolithography is a technique wherein a photosensitive resist is coated on a thin film to be patterned, and the exposed thin film portion is dry- or wet-etched after exposure and development through a photo mask, whereby patterning is carried out. After patterning, the unwanted resist is stripped, and formation of films thereon is repeated to form a semiconductor layer.
As described above, the photolithography has been used to produce an organic TFT element. However, this involves a problem of complicated production process and requirements for large-scale facilities such as a clean room. In recent years, to make up for the disadvantage of the organic TFT element manufacturing process based on the conventional photolithographic process, there is a very active effort going on for research and development of the organic TFT element using an organic semiconductor material (Japanese Laid-Open Patent Publication H10-190001, Journal of Advanced Material, 2002, No. 2, P. 99 (Review)).
The organic TFT can be manufactured, for example, as follows, although it depends on the type of element structures: In the case of a bottom gate-bottom contact structure wherein the gate electrode is formed on the substrate, a gate electrode is formed on the substrate using the photolithography. After that, a gate insulation film is formed on the gate electrode by plasma CVD using a TEOS source, or the coated insulation material is formed by printing.
Then the source electrode and drain electrode are formed by photolithography. After that, an organic semiconductor layer is formed on the channel area between the source electrode and drain electrode.
The organic semiconductor layer is formed by vapor deposition under vacuum. The problem is that the organic TFT having been manufactured is expensive due to high costs of the production equipment. To solve this problem, the film is formed by coating the dispersion solution obtained by dispersing the organic semiconductor material in a solvent, or the solution obtained by dissolving the organic semiconductor material. Spin coating method or inkjet method is used for this coating. Further, micro-contact printing method is also being studied.
Generally, when an organic semiconductor layer is formed, it is important that the semiconductor layer should be formed so as to minimize the contact resistance between the organic semiconductor layer and source electrode or drain electrode, thereby obtaining an organic TFT characterized by a higher degree of mobility and smaller variation.
To solve the aforementioned problem, the following method is proposed. The self-assembled monolayer of the thiol compound is formed on the surfaces of a source electrode and drain electrode. After that, the semiconductor portion is formed on the channel portion, whereby the contact resistance is reduced (for example, see the Official Gazettes of U.S. Pat. No. 6,335,539 and U.S. Pat. No. 6,569,707).
However, to form the self-assembled monolayer of thiol compound on the surfaces of the source electrode and drain electrode according to the method disclosed in the Official Gazettes of U.S. Pat. No. 6,335,539 and U.S. Pat. No. 6,569,707, it is necessary to take a step of immersing the source electrode and drain electrode in the thiol-containing solution and a step of drying the thiol. This takes a lot of time. Further, it is difficult to set the conditions for the processes of immersing and drying. Variations occur to the TFT characteristics due to a slight difference in the conditions. Such problems have been kept unsolved.